1. Field of the Invention
This invention relates to lubrication of hydrophobic surfaces. More particularly, the invention relates to a method for lubricating a hydrophobic polymeric surface to give an even coat of lubricant which is stable over a protracted period.
2. Background of the Invention
Many devices require lubrication of a surface. In the medical instrumentation and diagnostic field, simple sensing devices such as, for example, thermometers, needles or electrode components of complex monitoring apparatuses must be inserted into a body cavity or through the skin and at a later time withdrawn. Absent effective lubrication which is stable throughout both the insertion and withdrawal stages of the procedure, severe patient discomfort may result.
Other medical devices, such as syringes cannulas and catheters used for sampling or medicament administration, or devices such as burets used in diagnostic procedures, have components which are in sliding contact during use. Such devices require lubrication of the moving parts and may well also require lubrication of an external surface.
In the medical arts, synthetic polymers have come to the fore as materials of choice for fabrication of devices. Although polymers have many salubrious properties which make them useful in medical devices, such as flexibility and biocompatibility consequent to chemical inertness, they have the disadvantage of being materials of low surface energy. One of the otherwise most useful classes of polymers, the perfluorinated hydrocarbons, has the lowest surface energy of any known polymer class.
Lubrication of surfaces of low surface energy is a long-standing problem because of the propensity of lubricants to migrate from surface to surface interfaces or to "bead" on an external surface. Either phenomenon severely limits the effectiveness of a lubricant on a low-energy surfaces.
Migration and beading of a lubricant on a surface are believed to be related to the surface tension of both the lubricant and the surface, and are easily understood with a simple and familiar analogy. Anyone who has ever washed and polished a car has observed water to form discreet drops on the hood after polishing. The same situation may exist when a lubricating oil is applied to a surface. If the lubricant forms beads on the surface or migrates from a surface to surface interface, very ineffective lubrication results.
The antithesis of beading, i.e., the ability of a liquid to spread out and cover a surface is termed wettability and this property is measured by the conventional contact angle formed between the surface and a drop of the liquid applied to the surface. A high contact angle is indicative of beading. Conversely, a low contact angle indicates the desired wetting. Complete spreading giving a uniform coating of the liquid on the surface is indicated by the theoretical contact angle of 0.degree..
Many attempts to achieve wetting of a surface with a liquid by addition of various surfactants have been reported. This approach has been partially successful with some surfaces, but no surfactant is known which allows formation of a uniform stable coating of a liquid on a perfluorinated surface. Other surface treatments which have been tried are strong acids, oxidizing agents and flame treatments.
Achievement of wettability to water by treatment of polymeric surfaces with an ionizing plasma formed by electromagnetic activation of a gas by either glow discharge or corona discharge is well-known and is summarized by Rose et al. in Proceedings of the SPE 43rd Annual Technical Conference and Exhibition, 1985, page 685. Specific examples of enhancement of water wettability by plasma treatment of a polymeric surface are U.S. Pat. No. 4,445,991 to Arbit and U.S. Pat. No. 4,344,981 to Imada et al. The latter patent discloses that plasma treatment of a silicone surface gives water affinity of short duration, but that plasma treatment followed by treatment with an aqueous solution of a surface active agent gives water affinity of long duration.
U.S. Pat. No. 4,072,269 to Lidel discloses treatment of a polymeric surface with a plasma from an activator gas and a reactive gas whereby surface wettability to water is increased but wettability to an oil is decreased, thereby inhibiting penetration of the oil into the polymer.
Enhancement of ink receptivity rendering polymeric surfaces printable is achieved by plasma treatment in U.S. Pat. No. 4,292,397 to Takeuchi et al.
Auerbach, in U.S. Pat. No. 4,188,426, discloses plasma deposition of a fluorocarbon coating onto an organic or inorganic surface wherein the lubricity, hydrophobicity and coefficient of friction of the resulting fluorocarbon surface are equivalent to those provided by conventional fluorocarbon polymers.
U.S. Pat. No. 4,642,246 to Janssen et al. has recently disclosed an approach to overcoming the problem of loss of lubricant from a magnetic disk by heating a polymeric lubricant having a terminal functional group to covalently bond the lubricant to surface polymers of the disk.
Copending application Ser. No. 036,733, filed Apr. 10, 1987, and having a common assignee herewith, discloses plasma treatment of a surface and lubrication thereof in a method to overcome breakout.
In spite of the vast literature on lubrication, the problem of providing stable lubricated surfaces for devices fabricated from polymeric surfaces of low energy has not yet been solved. It is toward the solution of this long-standing problem that this invention is directed.